This implementation of MessageGroupProcessor will return all messages inside the group.
This is useful if there is no requirement to process the messages, but they should just be
blocked as a group until their ReleaseStrategy lets them pass through.

PassThroughMessageGroupProcessor

processMessageGroup

Process the given MessageGroup. Implementations are free to return as few or as many messages based on the
invocation as needed. For example an aggregating processor will return only a single message representing the
group, while a resequencing processor will return all messages whose preceding sequence has been satisfied.

If a multiple messages are returned the return value must be a Collection<Message>.